It is known in radio frequency systems to use a coaxial or waveguide transmission system to propagate radio frequency (RF) signals from place to place, such as from a broadcast transmitter to a broadcast antenna.
A disadvantage of systems in current use is that a waveguide or coaxial transmission system consisting of a transmission line and a load device can deteriorate gradually due to its exposure to a number of destructive factors, while the deterioration can continue undetected in normal use until a catastrophic failure reveals one or more problems. Since a transmission system may be used, for example, for broadcast of several television channels using a shared, broad-band antenna system, it is possible for a multiplicity of channel transmissions to be rendered inoperable or degraded to a significant extent for an extended time interval until repair parts can be procured and installed.
Destructive factors likely to contribute to harm to transmission systems over periods of months or years can include, as examples, galvanic corrosion; cold flow under bolts caused by repeated temperature swings; metal fatigue associated with repeated mechanical stress cycling; exposure to chemically reactive pollutants; exposure to airborne salt; exposure to water condensed inside the transmission system due to temporary loss of pressurization or dehumidification; and hidden workmanship errors such as pinched O-rings or pinhole flaws in the materials from which the transmission system is constructed.
These and many other factors can lead to significant increases in voltage standing wave ratio (VSWR) at a point, high voltage breakdown, and other faults demanding immediate attention. VSWR increases stemming from such destructive events can manifest suddenly due to events such as storm stress on deteriorated materials, or can manifest gradually in the form of cumulative degradation in performance over a protracted period. Both problem modalities can require extensive efforts simply to identify and localize a single-point failure even after it has occurred. Multiple-point degradations can likewise become established, requiring prolonged and potentially ineffective troubleshooting.
A typical operator response to prevent some types of severe functional faults from continuing to grow worse is prompt decrease in output power, resulting in loss of reception for the furthest customers and degraded reception for those nearer. Even this response is not diagnostic, so it is still necessary to identify the failure that caused the system fault. A time consuming diagnostic procedure can ensue, leading to the development of a corrective action plan, which may involve shutting down a multi-channel broadcast facility for an extended period during prime viewing hours, switching over to a reduced-performance backup system for an indefinite servicing period, or another method, any of which can potentially entail significant contract violations and incur financial penalties.
A transmitted RF test signal in the form of a particular embedded data pattern is specified by the Federal Communications Commission (FCC) as mandatory for National Television Standards Committee (NTSC) television, the 525-line, 30-interlaced-frames per second standard used in North and South America, Japan, Taiwan, and some other countries. The mandatory test pattern has been inserted into the signal in order to allow an analytical device in a television receiver to identify and eliminate so-called ghost images caused by reflections of a program channel that arrive later than the direct-path signal for that channel. While the Ghost Cancellation Reference signal (GCR) is available for use in receivers, no methodology has been established for incorporating GCR into a high-power NTSC RF broadcast transmission system for real-time analysis of the state of health of the transmission system. Similarly, no existing system has been established for realtime testing of non-broadcast applications.
Accordingly, there is a need for a detective and diagnostic apparatus and method that can be applied continuously to provide prompt detection of degradation taking place in an RF transmission system, to include identification of the locus of each of an indefinite number of incipient failures.